1. Field of the Invention
This invention pertains to the general field of optical metrology. In particular, it pertains to a novel method and apparatus for assisting the manual or automated process of finding best focus for an imaging optical system.
2. Description of the Prior Art
The process of placing a sample in optimal focal position for imaging purposes, such as for interferometric measurements, is well understood and has been implemented using a variety of approaches that are typically based on finding the position corresponding to maximum irradiance at the detector. However, the depth of focus of an imaging optical system such as an interferometric objective is often greater than the depth of fringes produced by it. Therefore, the sample may be in focus and exhibit a high level of irradiance at the detector even though interference fringes are not visible. In such situation an erroneous measurement or no measurement at all may result.
Even when operating within the depth of fringes, it is still necessary to know in what direction to move the objective (or the sample) in order to position it in the best focal position. Typically that is done by trial and error with the attendant time delays required to assess the current scanner position relative to best focus.
Another recurring problem lies in the fact that, as the sample approaches the best focus position, the irradiance at the detector increases and it may be so large as to saturate the detector, thereby preventing the accurate determination of the position corresponding to maximum irradiance. Thus, unless the operator has some a-priori knowledge, the setting of the appropriate level of intensity for the light source is problematic.
The present invention is directed at an approach that provides solutions to these problems directly and in a manner suitable for automation.